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Extract:

Nutritional epidemiology provides evidence about the
diet-disease relationship to inform Public Health Nutrition (PHN) and enable
the development of prevention methods [Boeing, 2013].

Almost 60% of global colorectal cancer (CRC) cases are seen
in developed countries [Ferlay et al., 2010]. In the UK, CRC is the fourth most
common cancer; accounting for an incidence rate of 47 per 100,000 in 2011 [CRUK,
2014a].

The WCRF/AICR [2007] recognise the complexities of
determining associations between diet and cancer, and conclude that red and
processed meat (RAPM) are ‘convincing’ causes of CRC. This is reinforced by
subsequent meta-analyses by Chan et al. [2011] and Xu et al. [2013]. In
contrast, Alexander and Cushing [2011] suggest there is inadequate evidence to
support this relationship .

The aim of this essay is to critically appraise two cohort
studies by Norat et al. [2005] and Cross et al. [2007], observing the
relationship between RAPM consumption and CRC risk. These studies have been
selected due to their large sample size and robust study design. Since CRC is largely influenced
by diet [Willett, 2005], understanding these influences will enable development
and implementation of PHN strategies at a national and local level [Webster-Gandy,
Madden and Holdsworth, 2012].

Critical Discussion

Study Design

Cohort studies are the strongest observational design for
diet and cancer [Gibney et al., 2004]. Although randomised controlled trials
(RCTs) are considered the ‘gold standard’ of quantitative research methods [Aveyard
and Sharp, 2009], they are not always appropriate [Morrow, 2010]. Since cancer
develops slowly, applying intervention trials can be difficult [Gibney et al.,
2004]. It may also be unethical to use a RCT, as some participants would not
receive the potentially beneficial treatment [Sanson-Fisher et al., 2007].
Cohort studies however are considered to be ethical, and are useful for establishing
a sequence of events [Heneghan and Badenoch, 2006]. In contrast, they do involve enrolling numerous
participants which is costly [Willett, 2013]. There is also potential for
selection bias, information bias and confounding bias which can reduce the
reliability and validity of the findings [Healey and Devane, 2011].

Confounding

Both studies used Cox regression to adjust for confounders.
CRUK [2014b] outline the dietary and lifestyle factors believed to be
associated with CRC risk. Both studies have failed to account for some of these
factors, which could lead to an inappropriate assumption that the exposure
increases disease risk [Healy and Devane, 2011].

In the XXXX study, cooking methods were not considered, even
though certain methods have been linked with carcinogen production [WCRF /AICR,
2007]. Genetic susceptibility was also not accounted for, despite the evidence
of an association between family history and CRC [Johns and Houlston, 2001].
Additionally, socioeconomic factors were not taken into account, despite the
positive association between deprivation and CRC incidence in males [PHE, 2014].

In the YYYY study, the highest consumers of red meat also had
other CRC risk factors. Therefore, residual confounding may have contributed to
the positive associations observed. Additionally, 42.8% of women reported use
of hormone replacement therapy (HRT), which wasn’t adjusted for, even though
HRT is believed to reduce CRC risk [CRUK, 2014b].

World Cancer Research Fund (WCRF)/American Institute for
Cancer Research (AICR) (2007) Food, nutrition, physical activity and the
prevention of cancer: a global perspective (pdf). Available at:
http://www.dietandcancerreport.org/cancer_resource_center/downloads/Second_Expert_Report_full.pdf

I am module lead for Nutritional Epidemiology module. This assessment is a challenge for the students as it tests their critical writing skills (and nutrition as a subject area that provides many complexities with this skill). This essay excerpt is taken from a student who achieved a first class mark for this assessment. This essay (of which excerpts are shown) was well structured, with good use of a range of referenced sources, which were well integrated to demonstrate good understanding of epidemiology concepts as well as excellent skills in critical appraisal of the evidence.